1. Field of the Invention
This invention relates to bleaching agents in granulated form containing a solid aliphatic peroxycarboxylic acid as the bleaching component.
2. Description of Related Art
Bleaches based on peroxygen compounds are widely used in the bleaching of fabrics. Commonly used bleaches of this type include, in particular, hydrogen peroxide and its inorganic derivatives, such as sodium perborate and sodium percarbonate, which on the one hand represent mild oxidizing agents that are very safe to handle and, on the other hand, show good bleaching power at sufficiently high temperatures. By contrast, bleaching at relatively low temperatures requires stronger oxidizing agents, such as peroxycarboxylic acids for example, in order to obtain bleaching in a sufficiently short time. However, peroxycarboxylic acids (also referred to in short as percarboxylic acids or simply per-acids) are very aggressive oxidizing agents which show a tendency towards exothermic decomposition and explosion and cannot be handled in pure form without precautionary measures. Accordingly, it has been proposed inter alia--with a view to overcoming these disadvantages--to produce these compounds in situ from the non-dangerous inorganic per compounds and certain acylating agents, so called activators, which as such do not undergo exothermic decomposition or oxidation reactions, although they are sensitive to hydrolysis. This process is complicated by the use of two components which have to be separately metered, in addition to which the activation reaction between the inorganic per compounds and the activators requires an undesirably long induction time in some cases, above all at very low working temperatures.
For these reasons, attempts have long been made to find suitable measures to enable percarboxylic acids to be used as such for bleaching fabrics, despite their instability and aggressive chemical properties. In this connection, attention has been focused on the solid percarboxylic acids, particularly for use in combination with washing processes.
Thus, Belgian Patent 560 389 describes the stabilization of solid peroxycarboxylic acids using hydratable inorganic salts, in which the product may also be granulated. Conversion into the granulated form is particularly desirable when the percarboxylic acids are to be mixed with other components which, for example because of their sensitivity to oxidation, must not come into direct contact with the per-acids. DE-OS 2422 691 (British 1456591) mentioned one particular form of stabilization with salts in which mixtures of magnesium sulfate with a little sodium or potassium sulfate are used.
Another measure for preventing undesirable interactions between peroxycarboxylic acids and other components is the coating of per-acid particles. In FR-PS 1 262 475 (Canadian Patent 615,777), hydrophilic film forming agents, such as gelatin, are used for this purpose. According to GB-PS 1 387 167, water-impermeable materials, such as fats and waxes, are used for coating. Another variant is described in DE-OS 27 37 864 (U.S. Pat. No. 4,126,573) in which surfactants are used as coating materials.
In addition, numerous proposals have been made to combine desensitization by salt hydrates with the coating process, as for example in DE-OS 24 22 735 (British 1456592) which describes a mixture of two granulates of which one consists of percarboxylic acid particles containing salt hydrates and coated with fatty alcohol. Similar granulates are described in U.S. Pat. Nos. 3,770,816 and 4,170,453 and in DE-OS 26 52 424 (U.S. Pat. Nos. 4,225,451 and 4,288,388). U.S. Pat. No. 4,259,201 provides an example of the use of salt-containing, surfactant-coated per-acid granulates in detergents.
Although numerous problems involved in the use of peroxycarboxylic acids have been solved by the described measures, a percarboxylic acid formulation which satisfies all requirements in regard to safe handling, mechanical and chemical stability, solubility and economic production is still a long way off. Thus, although the granulates of per-acids and inorganic salts are sufficiently desensitized, they lack abrasion resistance so that, in many cases, it is not possible to prevent release of the peroxycarboxylic acids from the granulates during storage and, hence, oxidation of other sensitive components of the bleach preparations containing these granulates. Although the use of coating materials improves the mechanical properties, it does give rise to disadvantages, such as poor chemical stability in the case of certain hydrophilic coating materials or the prevention of dissolution in water in the case of hydrophobic coatings or those of anhydrous surfactants.
Accordingly, efforts are still being made to find new per-acid formulations having better all-round properties. In this connection, attempts are being made inter alia to fathom the dependence of stability on the type of auxiliaries added. Thus, many complexing agents capable of masking heavy metals have proved effective stabilizers against catalytic decomposition in almost all peroxycarboxylic acids, whereas with certain peroxycarboxylic acids, for example, surfactants promote decomposition, as mentioned in U.S. Pat. No. 3,639,285. With other peroxycarboxylic acids, certain coating compositions have a destabilizing effect, as can be seen from EP-OS 74 730 (Canada 190441). On the strength of these and other similar results, current opinion would appear to be that experience with one type of per-acid can rarely be applied to another type. Accordingly, optimal formulations can only be achieved by measures that are individually tailored to the particular type of percarboxylic acid.